In our previously filed U.S. Ser. No. 07/533,213, we described improvements in ferroelectric materials that are sensitive to infrared radiation. U.S. Ser. No. 07/533,213 specifically discloses ferroelectric polycrystalline potassium tantalate niobate thin films having enhanced pyroelectric effect at ambient temperatures. U.S. Ser. No. 07/533,213 also describes a metallo-organic deposition process for making such films. The films are intended for use in a pyroelectric detector array for a staring-type infrared detector.
In U.S. Ser. No. 07/533,213, we point out that thermal imaging systems have been long used by the military as reliable aids for night vision surveillance. While thermal energies are most often used for night vision enhancement, such systems are also used in fog, haze, rain, and snow. It is, therefore, foreseeable that a vision enhancement system containing a thermal energy system might be useful in automotive applications as an effective driving aid in a variety of driving conditions. However, the vision enhancement system presently used by the military is a relatively expensive and inappropriate system for automotive applications.
Current state-of-the-art military imaging devices make use of various sensor materials and detector geometries. Some of the more successful materials in the past included Hg.sub.1-x Cd.sub.x Te, Pb.sub.2 Se and PtSi schottky diodes. Thermal imaging systems based upon these materials and technology have been demonstrated. Also, the manufacturability of such systems has been proven. However, the primary obstacles for the low-cost production of these vision enhancement systems appear to be the complex infrared (IR) scanning systems and the costly multi-stage cooling systems required to cool the sensing elements to their operating temperatures.
Therefore, it would be desirable to provide an alternative cost-effective detection system which would make use of an array of uncooled or only slightly cooled sensors in a staring-type, rather than scanning, system to detect infrared radiation, preferably in either the 3.5 micrometer or 8-12 micrometer wavelength range. By only slightly cooled, we mean that the sensors need not be maintained at cryogenic temperatures. Previously, the detectors for infrared vision enhancement systems have been primarily made from potassium tantalate niobate, KTa.sub.1-x Mb.sub.x O.sub.3 (KTN), barium strontium titanate, Ba.sub.1-x Si.sub.x Ti.sub.x O.sub.3 (BST), and modified lead zirconate titanate PbZr.sub.1-x Ti.sub.x O.sub.3 (PZT). By "modified" we mean to include lanthanum-doped PZT. These materials are similarly characterized by both the perovskite crystal structure in a large pyroelectric coefficient, making them useful as infrared detector materials.
Both the KTN and BST materials have Curie temperatures which may be varied by appropriately varying the ratios of their chemical constituents. Because of this property, the maximum infrared response (pyroelectric effect) of either KTN or BST may be adjusted to any ambient temperature, making them especially attractive for a high-volume, uncooled, i.e., non-cryogenic infrared detection system. Theoretical calculation based upon their pyroelectric coefficients, emissivities, volume specific heats, and other parameters suggest that the sensitivity of the KTN films should be a factor of about four more sensitive to infrared radiation than the BST films. Therefore, it appears that the use of KTN films is the preferred material for formation of relatively inexpensive, high-volume, uncooled infrared detectors. In the prior U.S. Ser. No. 07/533,213, we focused on KTN. We stated that a typical staring-type infrared detector could be made from an approximately 5 micrometer thick film of KTN having an array thereon of pixels approximately 15 micrometers square. We now believe that the film should be less than 30 microns thick, preferably less than 20 microns thick, and that 10 to 15 microns thick might be the best overall thickness to use. Less than about 5 micrometers thick would make the film too thin for handling purposes, if it is to be separated from the support on which it was formed.
Ideally, the ferroelectric film would preferably be formed from single crystal material for maximum detector response. However, as indicated above, a polycrystalline film may be used if the grain sizes, i.e., the average dimensions of monocrystalline particles, are appropriate. Ordinarily, the monocrystalline particles, i.e., grains, forming the polycrystalline film should at least be about the size of a single domain for the ferroelectric material involved, which in this case is KTN. On the other hand, the grain size in our polycrystalline film should not be so large as to occupy the total thickness of the film. Otherwise, film inhomogeneties, such as porosity, will exist. Accordingly, grain sizes of about the thickness of the film are to be avoided, or significant porosity is likely. Generally, one does not want a grain size that is greater than about one-half the thickness of the film, or porosity may get too high. On the other hand, it appears that in some cases, grain sizes up to two-thirds the thickness of the film might be satisfactory.
One might think that the preferred infrared pyroelectric detector array would consist of reticulated pixels formed on single crystal KTN material that is about 5 micrometers thick. However, this is impractical for high-volume production purposes due to the difficulties in (1) growing chemically uniform single crystals of KTN of sufficient diameter, (2) slicing and thinning the crystals to good parallelism when thinned to dimensions approaching 5-30 micrometers, without degrading crystal quality, and (3) reticulating the film to form the discrete or at least individual pixels. Reticulating the film, i.e., cutting it into individual pixels, is needed if one cannot make the film thin enough. If one can make a very thin film as, for example, less than 20 micrometers, one need not cut the film up. In other words, the pyroelectric effect will be sufficient to show up as a change in capacitance in the array, even though the electrodes are arranged on a continuous film. In this invention, one can easily form such thin films. Even though they use polycrystalline, they are of high quality, rivalling the type of quality achieved with single crystal films after they have been ground down from large thicknesses, and then cut up into discrete pixel areas.
In this connection, thinner is better from a pyroelectric effect standpoint. However, there are handling limitations of about 5 micrometers for our films. Quality in very thin single crystal films may be the limitation in how thin one can go. One has to consider the damage done to the single crystal in the grinding process used to thin a single crystal wafer.
Thus, it appears important that for the economical fabrication of an uncooled, i.e., non-cryogenic, infrared detector having maximum response, one needs a process that will: (1) deposit high-quality thin films of polycrystalline KTN material over relatively broad areas; (2) control the polycrystalline deposition to get monocrystalline grains of appropriate size; (3) obtain chemical, physical, and dimensional uniformity of the films over the dimensions of the detector array; and (4) deposit films of maximum appropriate grain size so as to most closely replicate the properties obtainable with single crystal material. However, in the past, previous attempts to deposit KTN films having these characteristics have proven unsuccessful, such as with the sputter deposition or chemical vapor deposition techniques.
In the prior U.S. Ser. No. 07/533,213, we considered it inventive to recognize that metallo-organic deposition techniques can be used to provide a polycrystalline film that would have the characteristics one desired for infrared detection. It was desired to have a film of uniform stoichiometry, not only across its face, but through its thickness. In our metallo-organic deposition (MOD) technique, a stoichiometric precursor of the desired film is deposited on a substrate, pyrolyzed on the substrate in air to convert the metallo-organic precursors to their stoichiometric constituent oxides, and the pyrolyzed intermediate product annealed to form the perovskite crystal structure throughout the film. We discovered that potassium evaporated from the film during anneal.
An important inventive improvement to the MOD process disclosed in the previously mentioned U.S. Ser. No. 07/533,213 was in starting with a somewhat potassium deficient film (from stoichiometry) at pyrolyzation and in then conducting the KTN anneal in a potassium-rich environment to produce stoichiometry in the annealed film. We believed that this resulted in films having uniform, high-quality physical, chemical, and electrical properties throughout their thickness.
We have now discovered that the films we actually made were slightly more deficient in potassium content (from stoichiometry) than we intended, and that the potassium anneal we described did not, in fact, produce a uniform potassium content through the thickness of the film. Our method, in fact, produced a potassium gradient on the surface of the film exposed to the potassium atmosphere.
In any event, the films we reported in our earlier filed U.S. Ser. No. 07/533,213 had peak dielectric constants ranging between 7,800 to about 11,000 at a Curie temperature of about 9.degree. C. with a corresponding pyroelectric coefficient, P.sub.f equal to about 1.22.times.10.sup.-8 C/cm.sup.2 -.degree.C. to a 5.0.times.10.sup.-8 C/cm.sup.2 -.degree.C. X-ray diffraction analysis confirmed the presence of the KTN perovskite structure in the films. Scanning electron microscopy revealed grain sizes of the films in the order of about 1-10 micrometers. Film thicknesses up to 30 micrometers were used. The KTN films formed in this manner were used for fabrication of an uncooled infrared detector.
We have now found an even more significant improvement in the manufacture of such films. It appears now that even greater benefits are obtained if the starting film composition is more closely, and preferably exactly, stoichiometric. In such instance, in-diffusion during annealing will make the resultant film, i.e., the annealed film, non-stoichiometric at its surface. It will be potassium-rich at its surface. Moreover, it appears that enhanced infrared detection in a staring-type of infrared detector (over that available from our method as described in our previously filed U.S. Ser. No. 07/533,213) can be realized from the resulting potassium surface gradient. Accordingly, a significant improvement over our teachings in U.S. Ser. No. 07/533,213 has been achieved for a conventional staring-type infrared detector. For example, in our U.S. Ser. No. 07/533,213, we report that a peak dielectric constant ranging between 7,800 to about 11,000 is produced. When the films having a potassium surface gradient over stoichiometry of the form hereinafter described, peak dielectric constants ranging up to 20,000 can be produced. Accordingly, the U.S. Ser. No. 07/533,213 process produces an improvement, but even more of an improvement is obtained if the pyrolyzed film (i.e., the starting film) is stoichiometric to start with, and not potassium deficient. Our currently preferred KTN film is stoichiometric in the majority of its thickness. However, at its surface, there is a higher potassium content that decreases with increasing distance into the thickness of the film. Since the film is on a thick substrate, it is believed that the increased but graded potassium content, over stoichiometry, is not present on the film surface adjacent the substrate. On the other hand, a gradient in potassium deficiency could be present, as will hereinafter be explained.
However, even more importantly, we have discovered that if a KTN film, especially a stoichiometric KTN film, has a graded increase in potassium content approaching its outer surface, the film exhibits an entirely new characteristic not previously recognized for this material. In fact, we believe that this new characteristic was not previously appreciated in any material.
The important new characteristic of our potassium-annealed KTN film is that the film exhibits an asymmetry in electrical characteristics. By this we mean that upon application of an alternating current voltage, the film exhibits a hysteresis. The hysteresis shows a DC voltage shift with increased AC voltage. This indicates that there is a significant charge accumulation occurring on the surface of the film.
In addition, under a constant alternating current voltage, energy input to the film from other sources, such as from infrared sources, also causes a DC voltage shift of the hysteresis. In such instance, under a constant AC potential, energy of one form, i.e., radiant energy, is converted to another form of energy, i.e., electrical charge. When this occurs, the film becomes a transducer. Even more importantly, our calculations and tests thus far indicate that the voltage shift produced by infrared radiation is far greater than the improvement in pyroelectric effect one can observe in a staring-type infrared detector. It appears that this new, i.e., hysteresis, effect can provide a new type of pyroelectric effect that is at least several orders of magnitude more sensitive than what is observed in a staring-type infrared detector formed with this material.
In fact, this graded increase in potassium content at the surface of a stoichiometric KTN film might even provide more startling results in a monocrystalline film of KTN.
In any event, it now appears that polycrystalline KTN films can be used as even more efficient infrared detectors if used in a type of detection system that would detect the hysteresis shift, rather than the capacitance change currently monitored in a staring type of infrared detector.
In addition, it appears that the graded increase in surface potassium content (over stoichiometry in the balance of the film) provides a graded dipole moment. Our studies make us believe that the characteristics inherent to this film would be widely applicable to any ferroelectric film, and perhaps to other materials as well. Not only can such a film be used as an infrared detector, but it can be used as a transducer, and as a rectifier.
It appears that under an AC field of 1-15 KHz, the ferroelectric film of this invention becomes charged on its opposite contact surfaces. It thus forms a unique capacitor. We refer to our new transducer as a transpacitor.